Rozprawy doktorskie: „Crack Propagation Analysis”

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胡思來 and See-loi Wu. "Numerical analysis of dynamic crack propagation." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1988. http://hub.hku.hk/bib/B31208459.

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Al-Falou, Ahmed Amir. "A theoretical analysis of crack propagation." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.624858.

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Wu, See-loi. "Numerical analysis of dynamic crack propagation /." [Hong Kong] : University of Hong Kong, 1988. http://sunzi.lib.hku.hk/hkuto/record.jsp?B12354569.

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Perng, Jia-Der. "Analysis of crack propagation in asphalt concrete using a cohesive crack model." Connect to this title online, 1989. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1094737677.

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Tilbrook, Matthew Thomas Materials Science &amp Engineering Faculty of Science UNSW. "Fatigue crack propagation in functionally graded materials." Awarded by:University of New South Wales. Materials Science & Engineering, 2005. http://handle.unsw.edu.au/1959.4/21885.

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Propagation of cracks in functionally graded materials (FGMs) under cyclic loading was investigated via experiments and finite element (FE) analysis. Alumina-epoxy composites with an interpenetrating-network structure and tailored spatial variation in composition were produced via a multi-step infiltration technique. Compressed polyurethane foam was infiltrated with alumina slip. After foam burn-out and sintering, epoxy was infiltrated into the porous alumina body. Non-graded specimens with a range of compositions were produced, and elastic properties and fatigue behaviour were characterised. An increase in crack propagation resistance under cyclic loading was quantified via a novel analytical approach. A simulation platform was developed with the commercial FE package ANSYS. Material gradient was applied via nodal temperature definitions. Stress intensity factors were calculated from nodal displacements near the crack-tip. Deflection criteria were compared and the local symmetry criterion provided the most accurate and efficient predictions. An automated mesh-redefinition algorithm enabled incremental simulation of crack propagation. Effects of gradient and crack-geometry parameters on crack-tip stresses were investigated, along with influences of crack-shape, crack-bridging, residual stresses and plasticity. The model provided predictions and data analysis for experimental specimens. Fatigue cracks in graded specimens deflected due to elastic property mismatch, concordant with FE predictions. In other FGMs, thermal or plastic properties may dominate deflection behaviour. Weaker step-interfaces influenced crack paths in some specimens; otherwise effects of toughness variation and gradient steps on crack path were negligible. Crack shape has an influence, but this is secondary to that of elastic gradient. Cracks in FGM specimens initially experienced increase in fatigue resistance with crack-extension followed by sudden decreases at step-interfaces. Bridging had a notable effect on crack propagation resistance but not on crack path. Similarly, crack paths did not differ between monotonic and cyclic loading, although crack-extension effects did. Recommendations for analysis and optimisation strategies for other FGM systems are given. Experimental characterization of FGMs is important, rather than relying on theoretical models. Opportunities for optimization of graded structures are limited by the properties of the constituent materials and resultant general crack deflection behaviour.

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Nielsen, Kristin. "Crack Propagation in Cruciform Welded Joints : Study of Modern Analysis." Thesis, Uppsala universitet, Institutionen för teknikvetenskaper, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-145134.

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This thesis is investigating how the effective notch method can be used for fatigue assessment of welded joints. The effective notch method is based on a finite element analysis where the joint is modeled with all notches fictitiously rounded with a radius of 1 mm. Analyses are performed on a cruciform fillet welded joint where parameters such as, load case, steel plate thickness and weld size, are varied. The achieved lifetime estimations are then compared to calculations with other fatigue assessment methods, linear elastic fracture mechanics and the nominal method. The goal is to draw conclusions about pros and cons of the effective notch method. The results are also compared to experimental fatigue tests performed on the same geometry. The results indicates that the effective notch method tends overestimating the lifetime, especially when the steel plate thickness is small. This leads to a non conservative method that is dangerous to use as guidance when designing. The estimations are though better when considering a toe crack then when considering a root crack. Due to a large scatter in experimental test results, it is hard to validate a fatigue assessment method in an absolute sense. That is also the case for the effective notch method, and more results from experimental fatigue tests are needed before the effective notch method can be fully used. For relative analysis, when variations of the same design needs to be compared, the effective notch can be a very powerful tool. This is because of the flexibility for different geometries that this method grants.

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Kim, Dong Sub. "Analysis of fatigue crack propagation behavior of a surface crack at fillet toe of tubular joints /." The Ohio State University, 1989. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487668215808373.

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Blandford, Robert. "Characterization of fatigue crack propagation in AA 7075-T651." Master's thesis, Mississippi State : Mississippi State University, 2001. http://library.msstate.edu/etd/show.asp?etd=etd-04092001-152127.

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Figiel, Lukasz. "Sensitivity Analysis of Interface Fatigue Crack Propagation in Elastic Composite Laminates." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2004. http://nbn-resolving.de/urn:nbn:de:swb:14-1102358246078-89903.

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Composite laminates are an important subject of modern technology and engineering. The most common mode of failure in these materials is probably interlaminar fracture (delamination). Delamination growth under applied fatigue loads usually leads to structural integrity loss of the composite laminate, and hence its catastrophic failure. It is known that several parameters can affect the fatigue fracture performance of laminates. These include the constituent material properties, composite geometry, fatigue load variables or environmental factors. The knowledge about effects of these parameters on fatigue delamination growth can lead to a better understanding of composite fatigue fracture behaviour. Effects of some of these parameters can be elucidated by undertaking appropriate sensitivity analysis combined with the finite element method (FEM) and related software. The purpose of this work was three-fold. The first goal was the elaboration and computational implementation of FEM-based numerical strategies for the sensitivity analysis of interface fatigue crack propagation in elastic composite laminates. The second goal of this work was the numerical determination and investigation of displacement and stress fields near the crack tip, contact pressures along crack surfaces, mixed mode angle, energy release rate and the number of cumulative fatigue cycles. The third aim of the present study was to use the developed strategies to evaluate numerically the sensitivity gradients of the total energy release rate and fatigue life with respect to design variables of the curved boron/epoxy-aluminium (B/Ep-Al) composite laminate in two different material configurations under cyclic shear of constant amplitude. This study provided novel strategies for undertaking sensitivity analysis of the delamination growth under fatigue loads for elastic composite laminates using the package ANSYS. The numerical results of the work shed more light on mechanisms of interfacial crack propagation under cyclic shear in the case of a curved B/Ep-Al composite laminate. Moreover, the outcome of the sensitivity gradients demonstrated some advantages for using the sensitivity analysis to pinpoint directions for the optimisation of fatigue fracture performance of elastic laminates. The strategies proposed in this work can be used to study the sensitivity of the interface fatigue crack propagation in other elastic laminates, if the crack propagates at the interface between the elastic and isotropic components. However, the strategies can be potentially extended to composites with interfacial cracks propagating between two non-isotropic constituents under a constant amplitude fatigue load. Finally, the strategies can also be used to undertake the sensitivity analysis of composite fatigue life with respect to variables of fatigue load.

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Echeverria, Molina Maria Ines. "Crack Analysis in Silicon Solar Cells." Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4311.

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Solar cell business has been very critical and challenging since more efficient and low costs materials are required to decrease the costs and to increase the production yield for the amount of electrical energy converted from the Sun's energy. The silicon-based solar cell has proven to be the most efficient and cost-effective photovoltaic industrial device. However, the production cost of the solar cell increases due to the presence of cracks (internal as well as external) in the silicon wafer. The cracks of the wafer are monitored while fabricating the solar cell but the present monitoring techniques are not sufficient when trying to improve the manufacturing process of the solar cells. Attempts are made to understand the location of the cracks in single crystal and polycrystalline silicon solar cells, and analyze the impact of such cracks in the performance of the cell through Scanning Acoustic Microscopy (SAM) and Photoluminescence (PL) based techniques. The features of the solar cell based on single crystal and polycrystalline silicon through PL and SAM were investigated with focused ion beam (FIB) cross section and scanning electron microscopy (SEM). The results revealed that SAM could be a reliable method for visualization and understanding of cracks in the solar cells. The efficiency of a solar cell was calculated using the current (I) - voltage (V) characteristics before and after cracking of the cell. The efficiency reduction ranging from 3.69% to 14.73% for single crystal, and polycrystalline samples highlighted the importance of the use of crack monitoring techniques as well as imaging techniques. The aims of the research are to improve the manufacturing process of solar cells by locating and understanding the crack in single crystal and polycrystalline silicon based devices.

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Sohrabi, Maria, and Loarte José Sanchez. "Crack propagation in concrete dams driven by internal water pressure." Thesis, KTH, Betongbyggnad, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-211648.

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Concrete structures are in general expected to be subjected to cracking during its service life. This is the reason why concrete is reinforced, where the reinforcement is only activated after cracks occur. However, cracks may be a concern in large concrete structures, such as dams, since it may result in reduced service life. The underlying mechanisms behind crack formations are well known at present day. On the other hand, information concerning the crack condition over time and its influence on the structure is limited, such as the influence of water pressure within the cracks. The aim of this project is to study crack propagation influenced by water pressure and to define an experimental test setup that allows for crack propagation due to this load. Numerical analyses have been performed on an initial cracked specimen to study the pressure along the crack propagation. The finite element method has been used as the numerical analysis tool, through the use of the software ABAQUS. The finite element models included in these studies are based on linear or nonlinear material behavior to analyze the behavior during a successively increasing load. The numerical results show that a crack propagates faster if the water is keeping up with the crack extension, i.e. lower water pressure is required to open up a new crack. When the water does not have time to develop within the crack propagation, more pressure is required to open up a new crack. The experimental results show that the connection between the water inlet and the specimen is heavily affected by the bonding material. In addition, concrete quality and crack geometry affects the propagation behavior.

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Ueda, Naoshi, Minoru Kunieda, Hikaru Nakamura, and Khoa Kim Tran. "Analysis of crack propagation behavior in concrete due to multi-rebar corrosion." 土木学会, 2012. http://hdl.handle.net/2237/20868.

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Kunaporn, Chalitphan. "Probabilistic Analysis of a Thin-walled Beam with a Crack." Diss., Virginia Tech, 2011. http://hdl.handle.net/10919/77072.

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It is reasonable to assume that an aircraft might experience some in-flight discrete source damage caused by various incidents. It is, thus, necessary to evaluate the impact of such damage on the performance of the aircraft. This study is focused on evaluating the effect of a simple discrete damage in an aircraft wing on its static and dynamic response. The damaged wing is modeled by a thin-walled beam with a longitudinal crack the response of which can be obtained analytically. As uncertainties are present in the location and size of the crack as well as in the applied loads, their effects are incorporated into the framework consisting of structural response, crack propagation and aeroelasticity. The first objective of this study is to examine the effect of damage represented by a crack on the wing flexibility that influences its deformation and aero-elastic divergence characteristics. To study this, the thin-walled beam is modeled by Benscoter thin-walled beam theory combined with Gunnlaugsson and Pedersen compatibility conditions to accurately account for the discontinuity at the interface of the cracked and uncracked beam segments. Instead of conducting a detailed finite element analysis, the solution is obtained in an exact sense for general distributed loads representing the wind pressure effects. This analytical approach is shown to provide very accurate values for the global beam response compared with the detailed finite element shell analysis. This analytical solution is, then, used to study the beam response probabilistically. The crack location and size are assumed to be uncertain and are, thus, characterized by random variable. For a specified limit state, the probability of failure can be conveniently calculated by the first order second moment analysis using the safety index approach. The same analytical solution is also used to study the aero-elastic divergence characteristics of a wing, the inner structure of which is represented by a thin-walled beam with a crack of uncertain size and position along the beam. The second objective of this study is to examine the time growth of a crack under dynamic gust type of loading to which a wing is likely to be exposed during flight. Damage propagating during operation further deteriorates the safety of the aircraft and it is necessary to study its time growth so that its impact on the performance can be evaluated before it reaches its unstable state. The proposed framework for the crack growth analysis is based on classical fracture mechanics where the remaining flight time is obtained by Monte Carlo simulation in which various uncertainties are taken into account. To obtain equivalent cyclic loading required for crack growth analysis, random vibration analysis of the thin-walled beam is conducted for stochastic wind load defined by a gust load spectral density function. The probability of failure represented by the crack size approaching the critical crack size within the flight duration or the remaining flight time before a crack reaches its limiting value are obtained. This study with a simple representation of a wing and damage is anticipated to provide initial guidance for future studies to examine the impact of discrete source damage on the in-flight performance of the aircrafts, with the ultimate goal of minimizing the adverse effect and enhancing the safety of aircrafts experiencing damage. Ph. D.

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Din, Khafilah Binti. "Fatigue crack propagation under variable amplitude loading analysed by fracture mechanics and finite element analysis." Thesis, University of Manchester, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334012.

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Hashtroodi, Seyedowjan. "Crack Propagation Analysis of a Pre-stressed L-shaped Spandrel Parking Garage Beam." University of Toledo / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1420753991.

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Nešpůrek, Lukáš. "STOCHASTIC CRACK PROPAGATION MODELLING USING THE EXTENDED FINITE ELEMENT METHOD." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2010. http://www.nusl.cz/ntk/nusl-233900.

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Tato disertační práce vychází z výzkumu v rámci francouzsko-českého programu doktorátu pod dvojím vedením na pracovišti Institut français de mécanique avancée v Clermont-Ferrand a na Ústavu fyziky materiálu AV v Brně. Úvodní výzkumný úkol na brněnském pracovišti se zabýval numerickou analýzou pole napětí v okolí čela trhliny v tenké kovové fólii. Zvláštní pozornost byla zaměřena na vliv speciálního typu singularity v průsečíku čela trhliny s volným povrchem. Těžiště disertační práce spočívá v numerickém modelování a stochastické analýze problémů šíření trhlin se složitou geometrií v dvojrozměrném prostoru. Při analýze těchto problémů se dříve zřídka používaly numerické metody, a to z důvodu vysoké náročnosti na výpočtový čas. V této disertaci je ukázáno, že aplikací moderních metod numerické mechaniky a vhodných technik v analýze spolehlivosti lze tyto problémy řešit s pomocí numerických metod i na PC. Ve spolehlivostní analýze byla využita lineární aproximační metoda FORM. Pro rychlost šíření trhlin se vycházelo z Parisova-Erdoganova vztahu. Pro parametry tohoto vztahu byl použit dvourozměrný statistický model, který postihuje vysokou citlivost na korelaci obou parametrů. Mechanická odezva byla počítána rozšířenou metodou konečných prvků (XFEM), která eliminuje výpočetní náročnost a numerický šum související se změnou sítě v klasické metodě konečných prvků. Prostřednictvím přímé diferenciace bylo odvozeno několik vztahů pro derivace funkce odezvy, čímž se dosáhlo lepší numerické stability a konvergence spolehlivostní analýzy a výrazného zkrácení doby výpočtu. Problém zatížení s proměnou amplitudou byl řešen aplikací transformace zatížení metodou PREFFAS. Využití distribuce výpočtů v síti PC umožnilo další zrychlení analýzy.

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Nilsen, Oda Kulleseid. "Simulation of crack propagation using isogeometric analysis applied with NURBS and LR B-splines." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for matematiske fag, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-19545.

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This report features the isogeometric finite element method applied to the elastodynamic problem in a brittle medium with a potential for cracking. Griffith's theory for fracturing is used. The development of the model is outlined, complete with the Euler-Lagrange equations. The cracking is described with a phase field supplemented with a history field, contrary to the usual way of building the crack directly into the geometry by modification of the basis, facilitating the use of isogeometric analysis even with simplistic basis functions such as Non-Uniform Rational B-Splines (NURBS). The introduction of the crack-phase field results in non-linearity in the coupled problem. The problem is semi-discretized, upon which the spatial sub-problem is treated with isogeometric analysis. The numerical time-stepping solution routine is built around the Newton-Raphson method, but includes both pre-conditioning and correctors and is known as the predictor/multi-corrector time integration scheme. The Jacobian of the semi-discretized system (needed for the Newton-Raphson iteration) is developed analytically. In addition to the numerical tests with NURBS as our basis, we will also test the method with Locally Refined B-splines (LR B-Splines), ensuring better resolution along the crack path. The LR B-spline represents an alternative to the more commonly used T-Spline.

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Prawoto, Yunan. "LEFM based analysis of the effect of tensile residual macrostress on fatigue crack propagation /." free to MU campus, to others for purchase, 2000. http://wwwlib.umi.com/cr/mo/fullcit?p9974672.

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Butt, Ali. "Dynamic calibration and analysis of crack tip propagation in energetic materials using real-time radiography." Thesis, The University of Alabama in Huntsville, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=1603134.

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Crack propagation in a solid rocket motor environment is difficult to measure directly. This experimental and analytical study evaluated the viability of real-time radiography for detecting bore regression and propellant crack propagation speed. The scope included the quantitative interpretation of crack tip velocity from simulated radiographic images of a burning, center-perforated grain and actual real-time radiographs taken on a rapid-prototyped model that dynamically produced the surface movements modeled in the simulation. The simplified motor simulation portrayed a bore crack that propagated radially at a speed that was 10 times the burning rate of the bore. Comparing the experimental image interpretation with the calibrated surface inputs, measurement accuracies were quantified. The average measurements of the bore radius were within 3% of the calibrated values with a maximum error of 7%. The crack tip speed could be characterized with image processing algorithms, but not with the dynamic calibration data. The laboratory data revealed that noise in the transmitted X-Ray intensity makes sensing the crack tip propagation using changes in the centerline transmitted intensity level impractical using the algorithms employed.

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Hütter, Geralf. "Multi-scale simulation of crack propagation in the ductile-brittle transition region." Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2013. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-121281.

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In the present thesis the crack propagation in the ductile-brittle transition region is studied on two scales with deterministic models. In the macroscopic model the ductile failure is described by a non-local Gurson-model whereas the discrete void microstructure is resolved around the crack tip in the microscopic model. The failure by cleavage is not evaluated by means of a post-processing criterion but is modeled equivalently using a cohesive zone model on both scales. Thus, cleavage is not a priori identified with unstable crack propagation but the transition between stable and unstable mode of propagation is a result of the simulation. The problem of handling completely failed material within the framework of non-local damage models is pointed out. A method to overcome this problem is proposed and successfully applied. The case of contained plastic yielding at the crack tip is addressed with a modified-boundary layer model. The macroscopic simulations reproduce many features which are known from experiments like the formation of stretch zones, cleavage after initial ductile tearing, pop-ins with crack arrest, among others. The microscopic simulations substantiate the understanding of the macroscopically observed behavior. Systematic parameter studies are performed. Starting with considerations on the limit cases like pure ductile failure or the lower-ductile brittle transition region allows to separate the effects of the different constitutive parameters. Based on these results, a methodology is proposed to extract the macroscopic material parameters from experiments. This scheme is successfully applied to experimental data from literature. The results show that the behavior of a low-constraint specimen can be reliably predicted with the parameters extracted from a high-constraint specimen In der vorliegenden Arbeit wird die Rissausbreitung im spröd-duktilen Übergangsbereich auf zwei Skalen mittels deterministischer Modelle untersucht. Das duktile Versagen wird im makroskopischen Modell durch ein nichtlokales Gurson-Modell beschrieben, während im mikroskopischen Modell die Porenmikrostruktur im Bereich um die Rissspitze diskret aufgelöst wird. Das mögliche Versagen durch Spaltbruch wird nicht, wie üblich, nachträglich durch ein spannungsbasiertes Kriterium bewertet. Stattdessen wird der Spaltbruch auf beiden Skalen durch ein Kohäsivzonenmodell abgebildet. Somit wird die Spaltbruchinitiierung nicht a priori mit instabiler Rissausbreitung gleichgesetzt. Vielmehr ist die Stabilität der Rissausbreitung ein Ergebnis der Simulationen. Außerdem wird das Problem der der Handhabung vollständig ausgefallenen Materials im Rahmen nichtlokaler Schädigungsmodelle herausgestellt. Es wird eine Methode vorgestellt, dieses Problem zu behandeln und erfolgreich angewendet. In den Simulationen wird der Fall vollständig eingebetteten, plastischen Fließens untersucht. Die Simulationen mit dem makroskopischen Modell geben viele Effekte wieder, die aus Experimenten bekannt sind. Dazu zählen die Ausbildung von Stretchzonen, die Spaltbruchinitiierung nach anfänglichem, duktilem Reißen oder lokale Instabilitäten mit Rissarrest. Die mikroskopischen Simulationen tragen zum Verständnis des makroskopisch beobachteten Verhaltens bei. In der vorliegenden Arbeit werden systematische Parameterstudien durchgeführt. Zunächst werden Grenzfälle wie das rein duktile Versagens oder der Spaltbruch in Abwesenheit der Mikroporen untersucht, um die Einflüsse der einzelnen Materialparameter abzugrenzen. Ausgehend von diesen Ergebnissen wird eine Prozedur vorgeschlagen, die Materialparameter des makroskopischen Modells Schritt für Schritt aus Experimenten zu bestimmen. Diese Prozedur wird erfolgreich auf experimentelle Daten aus der Literatur angewendet. Die Ergebnisse zeigen, dass es das entwickelte Modell erlaubt, das Verhalten einer Bruchmechanikprobe mit geringer Dehnungsbehinderung an der Rissspitze mit denjenigen Materialparametern vorherzusagen, die an Proben mit einer hohen Dehnungsbehinderung ermittelt wurden

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Rouault, Thomas. "Propagation de coupure en fatigue sur composites tissés – Etude expérimentale et modélisation." Thesis, Toulouse, ISAE, 2013. http://www.theses.fr/2013ESAE0018/document.

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Les pales d’hélicoptère sont des structures composites soumises à un chargement cyclique multiaxial, et leur criticité impose de porter une attention particulière à la tolérance aux dommages. Leur revêtement peut potentiellement présenter des criques suite à certains évènements (impact, défaut, foudre). Ces travaux se focalisent sur un matériau de revêtement donné (tissu de verre) et concernent l’étude de la propagation de coupure (crique) sous chargement cyclique. Les sollicitations de service ont amené à considérer la traction et le cisaillement plan. Une étude expérimentale a été menée afin d’étudier les modes d’endommagement du matériau et sa résistance à la propagation de coupure pour différentes sollicitations (en traction et en cisaillement) et pour les drapages les plus courants. Elle a permis de dégager les mécanismes d’endommagement mis en jeu, et a fourni un ensemble important de propriétés matériau et de données quantitatives de vitesse de propagation. Elle a par ailleurs guidé vers une modélisation par éléments finis adaptée à l’architecture du matériau, et la manière dont il se dégrade en fatigue. Ce modèle repose sur un maillage à l’échelle de la mèche, et la prédiction de la propagation est obtenue par l’utilisation d’une courbe de fatigue S-N. La simulation a été évaluée par comparaison des faciès de rupture, des vitesses de propagation et de l’étendue des zones d’endommagement avec les essais réalisés sur éprouvettes Helicopter blades consist of composite structures which have to sustain multi-axial cyclic loading. Because of their criticality, damage tolerance has to be considered carefully. Their skin is subjected to environmental events like impact, flaw, lightning which can cause through-thethickness cracks. The present work focuses on one given skin material (woven glass fabric) and concerns the study of the through-the-thickness crack growth under cyclic loading. In-flight loading lead to consider tension and shear. An experimental study has been carried out to study damage in the material and its crackgrowth resistance under different loadings (tension and shear) and for usual stacking sequences. It highlighted damage mechanisms and provided an important set of material data and crack growth speeds. Besides, this led to a finite element approach adapted to the woven fabric architecture, anddamage feature under fatigue loading. This modeling is based on a bundle scale mesh, a semidiscrete damage modeling and an S-N curve to predict fiber failure. Numerical simulations of crack growth tests were carried out, and results were compared with experiments in terms of crack direction, crack growth speed, and size of damaged area

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Karedla-Ravi, Shankar. "Modeling of crack tip high inertia zone in dynamic brittle fracture." Texas A&M University, 2003. http://hdl.handle.net/1969.1/5783.

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A phenomenological cohesive term is proposed and added to an existing cohesive constitutive law (by Roy and Dodds) to model the crack tip high inertia region proposed by Gao. The new term is attributed to fracture mechanisms that result in high energy dissipation around the crack tip and is assumed to be a function of external energy per volume input into the system. Finite element analysis is performed on PMMA with constant velocity boundary conditions and mesh discretization based on the work of Xu and Needleman. The cohesive model with the proposed dissipative term is only applied in the high inertia zone i.e., to cohesive elements very close to the crack tip and the traditional Roy and Dodds model is applied on cohesive elements in the rest of the domain. It was observed that crack propagated in three phases with a speed of 0.35cR before branching, which are in good agreement with experimental observations. Thus, modeling of high inertia zone is one of the key aspects to understanding brittle fracture.

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Fu, Chaoran, and Bjartmar Hafliðason. "Progressive failure analyses of concrete buttress dams : Influence of crack propagation on the structural dam safety." Thesis, KTH, Betongbyggnad, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-169707.

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Concrete buttress dams are the most common type of concrete dams for hydropower production in Sweden. Cracks have been observed in some of the them. However, only limited research has been made concerning the influence of these cracks on the structural dam safety. In conventional analytical stability calculations, a concrete dam is assumed to be a rigid body when its safety is verified. However, when cracks have been identified in a dam structure, the stability may be influenced and hence the information of cracks may need to be included in the stability calculations. The main aim of this project is to study how existing cracks and further propagation of these cracks, influence the structural dam safety. Another important topic was to study suitable methods to analyse a concrete dam to failure. In addition, a case study is performed in order to capture the real failure mode of a concrete buttress dam. The case study that has been studied is based on a previous project presented by Malm and Ansell (2011), where existing cracks were identified in a 40m high monolith, as a result from seasonal temperature variations. Two similar models are analysed where one model is defined with an irregular rock-concrete interface, and the other with a horizontal interface. Analyses have been performed on both an uncracked concrete dam but also for the case where information regarding existing cracks, from the previous project, have been included in order to evaluate the influence of cracks on the dam safety. The finite element method has been used as the main analysis tool, through the use of the commercially available software package Abaqus. The finite element models included nonlinear material behaviour and a loading approach for successively increasing forces called overloading, when performing progressive failure analyses. The results show that existing cracks and propagation of these resulted, in this case, in an increased structural safety of the studied dam. Furthermore, an internal failure mode is captured. The irregular rock-concrete interface has a favourable effect on a sliding failure and an unfavourable effect on an overturning failure, compared to the case with the horizontal interface. Based on the results, the structural safety and the failure mode of concrete buttress dams are influenced by existing cracks. Although an increased safety is obtained in this study, the results do not necessarily apply for other monoliths of similar type. It is thus important that existing cracks are considered in stability analyses of concrete buttress dams. Lamelldammar är den vanligaste typen av betongdammar för vattenkraft produktion i Sverige. I vissa av dessa har sprickbildning observerats. Begränsad forskning har gjorts på hur dessa sprickor påverkar dammens säkerhet. I de vedertagna analytiska stabilitetsberäkningarna antas att betongdammar agerar som en stel kropp när man verifierar dess säkerhet. Befintliga sprickor i en damm kropp kan dock påverka dess stabilitet och kan därför behöva beaktas i stabilitetsberäkningarna. Huvudsyftet med detta projekt är att studera hur befintliga sprickor och dess propageringen påverkar dammsäkerheten. Ett annat viktigt syfte är att studera lämpliga metoder för att analysera en betongdamm till brott. Dessutom, genomförs en fallstudie i syfte att analysera ett verkligt brottförlopp av en lamelldamm. Fallstudien som utförs i detta projekt, baseras på ett tidigare projekt utfört av Malm and Ansell (2011), där befintliga sprickor identifierades i en monolit på 40m som ett resultat av temperaturvariationer. Två modeller med snarlik geometri har analyserats, där den ena är definierad med en med oregelbunden kontaktyta mellan berg och betong och den andra med en horisontell kontaktyta. Analyserna har utförts på dels en osprucken damm men även där information om befintliga sprickor från det tidigare projektet beaktas, i syfte att jämföra inverkan av sprickor på dammsäkerheten. Finita element metoden har använts som verktyg vid dessa analyser, genom det kommersiellt använda programmet Abaqus. De finita element modellerna inkluderar icke-linjära material egenskaper hos betong och armering samt baseras på en metod för successiv belastning, som kallas 'overloading', vid analys av brottförloppet. Resultatet visar att befintliga sprickor och propageringen av dessa i detta fall kan leda till ökad säkerhet hos den studerade dammen jämfört mot fallet utan beaktande av sprickbildning. Utöver detta fångas även ett inre brottmod. Den oregelbundna kontaktytan mellan betongen och berget har en gynnsam effekt vid ett glidbrott men en ogynnsam inverkan vid ett stjälpningsbrott, i jämförelse med fallet med en horisontell kontaktyta. Baserat på dessa resultat så påverkas dammens säkerhet och brottetförloppet hos lamelldammen utav befintliga sprickor. Även om en ökad säkerhet fås i denna studie är det inte säkert att detta stämmer för andra monoliter av samma slag. Dock är det viktigt att hänsyn tas till befintliga sprickor i stabilitets analyser av lamelldammar.

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24

Anyintuo, Thomas Becket. "Seepage-Coupled Finite Element Analysis of Stress Driven Rock Slope Failures for BothNatural and Induced Failures." Scholar Commons, 2019. https://scholarcommons.usf.edu/etd/7731.

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Rock slope failures leading to rock falls and rock slides are caused by a multitude of factors, including seismic activity, weathering, frost wedging, groundwater and thermal stressing. Although these causes are generally attributed as separate causes, some of them will often act together to cause rock slope failures. In this work, two of the above factors, seepage of water through cracks and crack propagation due to the after effects of blasting are considered. Their combined impact on the development of rock falls and rock slides is modeled on ANSYS workbench using the Bingham Canyon mine slope failure of 2013 as a case study. Crack path modeling and slope stability analysis are used to show how a combination of crack propagation and seepage of water can lead to weakening of rock slopes and ultimate failure. Based on the work presented here, a simple approach for modeling the development of rock falls and rock slides due to crack propagation and seepage forces is proposed. It is shown how the information from remote sensing images can be used to develop crack propagation paths. The complete scope of this method involves demonstrating the combination of basic remote sensing techniques combined with numerical modeling on ANSYS workbench.

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25

Malm, Richard. "Predicting shear type crack initiation and growth in concrete with non-linear finite element method." Doctoral thesis, KTH, Bro- och stålbyggnad, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-10156.

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In this thesis, the possibility to numerically describing the behaviour that signifies shear type cracking in concrete is studied. Different means for describing cracking are evaluated where both methods proposed in design codes based on experiments and advanced finite element analyses with a non-linear material description are evaluated. It is shown that there is a large difference in the estimation of the crack width based on the calculation methods in design codes. The large difference occurs due to several of these methods do not account for shear friction in the crack face. The finite element method is an important tool for analysing the non-linear behaviour caused by cracking. It is especially of importance when combined with experimental investigations for evaluating load bearing capacity or establishing the structural health. It is shown that non-linear continuum material models can successfully be used to accurately describe the shear type cracking in concrete. A method based on plasticity and damage theory was shown to provide accurate estimations of the behaviour. The methods based on fracture mechanics with or without inclusion of damage theory, overestimated the stiffness after crack initiation considerably. The rotated crack approach of these methods gave less accurate descriptions of the crack pattern and underestimated the crack widths. After verification of the material model, realistic finite element models based on plasticity and damage theory are developed to analyse the cause for cracking in two large concrete structures. The Storfinnforsen hydropower buttress dam is evaluated where the seasonal temperature variation in combination with the water pressure have resulted in cracking. With the numerical model the cause for cracking can be explained and the crack pattern found in-situ is accurately simulated. The model is verified against measurements of variation in crest displacement and crack width with close agreement. The construction process of a balanced cantilever bridge, Gröndal Bridge, is numerically simulated and a rational explanation of the cause for cracking is presented. It is shown that large stresses and micro-cracks develop in the webs during construction, especially after tensioning the continuing tendons in the bottom flange. Further loads from temperature variation cause cracking in the webs that is in close agreement with the cracking found in-situ. The effect of strengthening performed on this bridge is also evaluated where the vertical Dywidag tendons so far seem to have been successful in stopping further crack propagation. QC 20100730

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26

Ozcan, Riza. "Fatigue And Fracture Analysis Of Helicopter Fuselage Structures." Master's thesis, METU, 2013. http://etd.lib.metu.edu.tr/upload/12615497/index.pdf.

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In this study a methodology is developed for the fatigue and fracture analysis of helicopter fuselage structures, which are considered as the stiffened panels. The damage tolerance behavior of the stiffened panels multiaxially loaded is investigated by implementing virtual crack closure technique (VCCT). Validation of VCCT is done through comparison between numerical analysis and the studies from literature, which consists of stiffened panels uniaxially loaded and the panel with an inclined crack. A program based on Fortran programming language is developed to automate the crack growth analysis under mixed mode conditions. The program integrates the prediction of the change in crack propagation direction by maximum circumferential stress criterion and the computation of energy release rate by VCCT. It allows reducing the computation time for damage tolerance evaluation for mixed mode cases through finite element analysis and runs the procedure file of MSC.Marc/Mentat for numerical analysis and the program generated by Patran Command Language (PCL) of MSC.Patran for remeshing. The developed code is verified by comparing the crack growth trajectories obtained by numerical analysis with the experimental studies from literature. A submodeling technique is utilized to analyze a particular fuselage portion of helicopter tail boom. Effects of different skin/stringer configurations of the helicopter fuselage structure on stress intensity factor are studied by means of the developed program. Fatigue crack growth analysis is performed by using stress intensity factors obtained from numerical analysis and fatigue propagation models proposed in literature.

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27

Davis, Timothy [Verfasser], Torsten [Akademischer Betreuer] Dahm, and Eleonora [Akademischer Betreuer] Rivalta. "An analytical and numerical analysis of fluid-filled crack propagation in three dimensions / Timothy Davis ; Torsten Dahm, Eleonora Rivalta." Potsdam : Universität Potsdam, 2021. http://d-nb.info/1236786645/34.

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Punnam, Pradeep Reddy, and Chitendar Reddy Dundeti. "A Finite Element Analysis of Crack Propagation in Interface of Aluminium Foil - LDPE Laminate During Fixed Arm Peel Test." Thesis, Blekinge Tekniska Högskola, Institutionen för maskinteknik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:bth-14100.

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This thesis deals with numerical simulation of a peel test with an Aluminium foil and Low Density Poly-Ethylene (LDPE) laminate. This work investigates the effects of the substrate thickness and studies the influences of interfacial strength and fracture energy of the cohesive zone between the Aluminium and LDPE. This study evaluates the proper guidelines for defining cohesive properties. A numerical cohesive zone model was created in ABAQUS. Continuum tensile tests were performed to extract LDPE material properties. The aluminium properties were found in literature. After acquiring material parameters, the simulation continued with studying the effects of changing interfacial strength, geometric parameters and fracture energy. The results were obtained in the form of root rotations and the force displacement response was studied carefully. It was validated by comparison to the traction separation curve.

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Li, Simin. "Cutting of cortical bone tissue : analysis of deformation and fracture process." Thesis, Loughborough University, 2013. https://dspace.lboro.ac.uk/2134/13411.

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Cortical bone tissue - one of the most intriguing materials found in nature - demonstrate some fascinating behaviours that have attracted great attention of many researchers from all over the world. In contrast to engineering materials, bone has its unique characters: it is a material that has both sufficient stiffness and toughness to provide physical support and protection to internal organs and yet adaptively balanced for its weight and functional requirements. Its structure and mechanical properties are of great importance to the physiological functioning of the body. Still, our understanding on the mechanical deformation processes of cortical bone tissue is rather limited. Penetration into a bone tissue is an intrinsic part of many clinical procedures, such as orthopaedic surgery, bone implant and repair operations. The success of bone-cutting surgery depends largely on precision of the operation and the extent of damage it causes to the surrounding tissues. The anisotropic behaviour of cortical bone acts as a distinctive protective mechanism and increases the difficulty during cutting process. A comprehensive understanding of deformation and damage mechanisms during the cutting process is necessary for improving the operational accuracy and postoperative recovery of patients. However, the current literature on experimental results provides limited information about processes in the vicinity of the cutting tool-bone interaction zone; while; numerical models cannot fully describe the material anisotropy and the effect of damage mechanisms of cortical bone tissue. In addition, a conventional finite-element scheme faces numerical challenges due to large deformation and highly localised distortion in the process zone. This PhD project is aimed at bridging the gap in current lack of understanding on cutting-induced deformation and fracture processes in the cortical bone tissue through experimental and numerical approaches. A number of experimental studies were accomplished to characterise the mechanical behaviour of bovine cortical bone tissue and to analyse deformation and damage mechanisms associated with the cutting process II along different bone axes in four anatomic cortices, namely, anterior, posterior, medial and lateral. These experiments included: (1) a Vickers hardness test to provide initial assessments on deformation and damage processes in the cortical bone tissue under a concentrated compressive load; (2) uniaxial tension and compression tests, performed to understand the effect of orientation and local variability of microstructure constituents on the macroscopic material properties of cortical bone; (3) fracture toughness tests, aimed at elucidating the anisotropic character of fracture toughness of cortical bone and its various fracture toughness mechanisms in relation to different orientations; (4) penetration tests, conducted to evaluate and validate mechanisms involved in bone cutting as well as orientation associated anisotropic deformation and damage processes at various different cortex positions. Information obtained in these experimental studies was used to assist the development of advanced finite-element models: (1) the effective homogenised XFEM models developed in conjunction with three-point bending test to represent a macroscopically, anisotropic elasticplastic fracture behaviour of cortical bone tissue; (2) three microstructured XFEM models to further investigate the effect of the randomly distributed microstructural constituents on the local fracture process and the variability of fracture toughness of cortical bone; (3) a novel finite-element modelling approach encompassing both conventional and SPH elements, incorporating anisotropic elastic-plastic material properties and progressive damage criteria to simulate large deformation and damage processes of cortical bone under penetration. The established models can adequately and accurately reflect large deformations and damage processes during the penetration in bone cutting. The results of this study made valuable contributions to our existing understanding of the mechanics of cortical bone tissue and most importantly to the understanding of its mechanical behaviours during the cutting process.

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Aure, Temesgen W. "Numerical Analysis of Cracking in Concrete Pavements Subjected to Wheel Load and Thermal Curling." University of Cincinnati / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1382427417.

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Shibanuma, Kazuki. "Reformulation of XFEM and its application to fatigue crack simulations in steel structures." 京都大学 (Kyoto University), 2010. http://hdl.handle.net/2433/120941.

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32

Bayraktar, Funda Seniz [Verfasser], and Norbert [Akademischer Betreuer] Huber. "Analysis of residual stress and fatigue crack propagation behaviour in laser welded aerospace Aluminium T-joints / Funda Seniz Bayraktar. Betreuer: Norbert Huber." Hamburg-Harburg : Universitätsbibliothek der Technischen Universität Hamburg-Harburg, 2012. http://d-nb.info/1048542289/34.

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Bayraktar, Funda Seniz Verfasser], and Norbert [Akademischer Betreuer] [Huber. "Analysis of residual stress and fatigue crack propagation behaviour in laser welded aerospace Aluminium T-joints / Funda Seniz Bayraktar. Betreuer: Norbert Huber." Hamburg-Harburg : Universitätsbibliothek der Technischen Universität Hamburg-Harburg, 2012. http://nbn-resolving.de/urn:nbn:de:gbv:830-tubdok-11318.

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Ouagne, Pierre. "Fracture property changes with oxidation and irradiation in nuclear graphites." Thesis, University of Bath, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.341675.

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Carvalho, Marcelo de Rezende. "Computational Framework for Fracture Simulation of Concrete Structures until Failure." Thesis, University of Sydney, 2019. https://hdl.handle.net/2123/23283.

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The need to predict the fractured behaviour of a structure with a high degree of certainty is becoming a significant problem in the construction industry, whether for designing new structures or for assessing and strengthening existing structures. Considerable advances in the construction industry – with the introduction of new materials and technologies and constant demand for safer, more cost-efficient, sustainable and bold designs – are overturning established design rules. It is becoming critical to bring new predictive tools to assure the safety and serviceability of these structures, and to accomplish the full potential of the new construction designs that are now becoming possible. This research developed a computational framework based on the discrete crack approach that can be efficiently used in engineering for the reliable simulation of the behaviour of concrete structures. The framework is built on an object-oriented finite element platform, specifically tailored to accommodate embedded strong discontinuities, and having tools to improve the simulation of discrete models, such as a non-iterative solution algorithm and a powerful direct sparse solver. Different new formulations are proposed for simulating and capturing crack propagation with embedded discontinuities, which: i) are based on local degrees of freedom, ii) are combined with embedded steel fibres, and iii) require minimum enhanced global degrees of freedom. Multiple case studies are performed for the validation of the new proposed techniques against important laboratory benchmark tests. The framework enables a close-to-reality prediction of the structural behaviour of plain, steel reinforced, and steel fibre reinforced concrete, with improved performance and without convergence issues in fracture simulations.

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Wang, Xiaofeng. "Computational technology for damage and failure analysis of quasi-brittle materials." Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/computational-technology-for-damage-and-failure-analysis-of-quasibrittle-materials(a7c91eb6-5058-4e73-95de-b2f3efd645d2).html.

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The thesis presents the development and validation of novel computational technology for modelling and analysis of damage and failure in quasi-brittle materials. The technology is demonstrated mostly on concrete, which is the most widely used quasi-brittle material exhibiting non-linear behaviour. Original algorithms and procedures for generating two-dimensional (2D) and three-dimensional (3D) heterogeneous material samples are developed, in which the mesoscale features of concrete, such as shape, size, volume fraction and spatial distribution of inclusions and pores/voids are randomised. Firstly, zero-thickness cohesive interface elements with softening traction-separation relations are pre-inserted within solid element meshes to simulate complex crack initiation and propagation. Monte Carlo simulations (MCS) of 2D and 3D uniaxial tension tests are carried out to investigate the effects of key mesoscale features on the fracture patterns and load-carrying capacities. Size effect in 2D concrete is then investigated by finite element analyses of meso-structural models of specimens with increasing sizes. Secondly, a 3D meso-structural damage-plasticity model for damage and failure analysis of concrete is developed and applied in tension and compression. A new scheme for identifying interfacial transition zones (ITZs) in concrete is presented, whereby ITZs are modelled by very thin layers of solid finite elements with damage-plasticity constitutive relations. Finally, a new coupled method named non-matching scaled boundary finite element-finite element coupled method is proposed to simulate crack propagation problems based on the linear elastic fracture mechanics. It combines the advantage of the scaled boundary finite element method in modelling crack propagation and also preserves the flexibility of the finite element method in re-meshing. The efficiency and effectiveness of the developed computational technology is demonstrated by simulations of crack initiation and propagation problems.

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37

Robinson, Elinirina Iréna. "Filtering and uncertainty propagation methods for model-based prognosis." Thesis, Paris, CNAM, 2018. http://www.theses.fr/2018CNAM1189/document.

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Les travaux présentés dans ce mémoire concernent le développement de méthodes de pronostic à base de modèles. Le pronostic à base de modèles a pour but d'estimer le temps qu'il reste avant qu'un système ne soit défaillant, à partir d'un modèle physique de la dégradation du système. Ce temps de vie restant est appelé durée de résiduelle (RUL) du système.Le pronostic à base de modèle est composé de deux étapes principales : (i) estimation de l'état actuel de la dégradation et (ii) prédiction de l'état futur de la dégradation. La première étape, qui est une étape de filtrage, est réalisée à partir du modèle et des mesures disponibles. La seconde étape consiste à faire de la propagation d'incertitudes. Le principal enjeu du pronostic concerne la prise en compte des différentes sources d'incertitude pour obtenir une mesure de l'incertitude associée à la RUL prédite. Les principales sources d'incertitude sont les incertitudes de modèle, les incertitudes de mesures et les incertitudes liées aux futures conditions d'opération du système. Afin de gérer ces incertitudes et les intégrer au pronostic, des méthodes probabilistes ainsi que des méthodes ensemblistes ont été développées dans cette thèse.Dans un premier temps, un filtre de Kalman étendu ainsi qu'un filtre particulaire sont appliqués au pronostic de propagation de fissure, en utilisant la loi de Paris et des données synthétiques. Puis, une méthode combinant un filtre particulaire et un algorithme de détection (algorithme des sommes cumulatives) a été développée puis appliquée au pronostic de propagation de fissure dans un matériau composite soumis à un chargement variable. Cette fois, en plus des incertitudes de modèle et de mesures, les incertitudes liées aux futures conditions d'opération du système ont aussi été considérées. De plus, des données réelles ont été utilisées. Ensuite, deux méthodes de pronostic sont développées dans un cadre ensembliste où les erreurs sont considérées comme étant bornées. Elles utilisent notamment des méthodes d'inversion ensembliste et un observateur par intervalles pour des systèmes linéaires à temps discret. Enfin, l'application d'une méthode issue du domaine de l'analyse de fiabilité des systèmes au pronostic à base de modèles est présentée. Il s'agit de la méthode Inverse First-Order Reliability Method (Inverse FORM).Pour chaque méthode développée, des métriques d'évaluation de performance sont calculées dans le but de comparer leur efficacité. Il s'agit de l'exactitude, la précision et l'opportunité In this manuscript, contributions to the development of methods for on-line model-based prognosis are presented. Model-based prognosis aims at predicting the time before the monitored system reaches a failure state, using a physics-based model of the degradation. This time before failure is called the remaining useful life (RUL) of the system.Model-based prognosis is divided in two main steps: (i) current degradation state estimation and (ii) future degradation state prediction to predict the RUL. The first step, which consists in estimating the current degradation state using the measurements, is performed with filtering techniques. The second step is realized with uncertainty propagation methods. The main challenge in prognosis is to take the different uncertainty sources into account in order to obtain a measure of the RUL uncertainty. There are mainly model uncertainty, measurement uncertainty and future uncertainty (loading, operating conditions, etc.). Thus, probabilistic and set-membership methods for model-based prognosis are investigated in this thesis to tackle these uncertainties.The ability of an extended Kalman filter and a particle filter to perform RUL prognosis in presence of model and measurement uncertainty is first studied using a nonlinear fatigue crack growth model based on the Paris' law and synthetic data. Then, the particle filter combined to a detection algorithm (cumulative sum algorithm) is applied to a more realistic case study, which is fatigue crack growth prognosis in composite materials under variable amplitude loading. This time, model uncertainty, measurement uncertainty and future loading uncertainty are taken into account, and real data are used. Then, two set-membership model-based prognosis methods based on constraint satisfaction and unknown input interval observer for linear discete-time systems are presented. Finally, an extension of a reliability analysis method to model-based prognosis, namely the inverse first-order reliability method (Inverse FORM), is presented.In each case study, performance evaluation metrics (accuracy, precision and timeliness) are calculated in order to make a comparison between the proposed methods

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Hartmann, Christoph [Verfasser], Wolfram [Akademischer Betreuer] Volk, den Boogaard Ton [Gutachter] van, and Wolfram [Gutachter] Volk. "Spatio-temporal optical flow methods for process analysis : Robust strain, strain rate, and crack propagation measurement in shear cutting / Christoph Hartmann ; Gutachter: Ton van den Boogaard, Wolfram Volk ; Betreuer: Wolfram Volk." München : Universitätsbibliothek der TU München, 2020. http://d-nb.info/1240832680/34.

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Soares, Claudio Roberto. "Métodos numéricos aplicados a mecânica da fratura: avaliação da integridade estrutural de componentes nucleares." CNEN - Centro de Desenvolvimento da Tecnologia Nuclear, Belo Horizonte, 2009. http://www.bdtd.cdtn.br//tde_busca/arquivo.php?codArquivo=114.

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Nenhuma A tenacidade à fratura avalia a resistência à propagação de uma trinca e tornou-se uma importante propriedade a ser considerada em metodologias para avaliação de integridade estrutural de componentes mecânicos em geral. A principal metodologia para avaliação dos efeitos da perda de restrição à plasticidade (Teoria J-Q) representa um grande passo para a inclusão dos efeitos geométricos nos estudos da fratura. Alguns avanços têm sido propostos para a obtenção de um parâmetro para transferência de valores de tenacidade obtidos em laboratório para o componente estrutural real. Assim, neste trabalho teórico de pesquisa foram realizadas diversas análises numéricas 3D, com o objetivo de compreender os efeitos da perda de restrição à plasticidade sobre os valores de tenacidade à fratura. Parâmetros como tamanho de trinca e geometria do corpo-de-prova foram analisados para fornecer informações mais detalhadas sobre a perda de restrição à plasticidade. Esta análise revelou uma variação da integral J e do parâmetro Q com a espessura de cada corpo-de-prova. Novas curvas J-QA foram traçadas para a descrição do nível de restrição à plasticidade e para um ajuste dos valores mais realísticos de tenacidade à fratura. Também foram comparados os resultados com o código ASME, onde foi possível ver como o mesmo tem um conceito extremamente conservador. The fracture toughness is used to evaluate the resistance to the propagation of a crack and became an important property to be considered in methodologies for structural integrity evolution of mechanical components in general. The main methodology for assessing the crack-tip constraint (JQ Theory) represents a major step towards the inclusion of geometrical effects in studies of fracture. Some advances have been proposed for a parameter to transfer values of toughness obtained in laboratory to real structural components. Thus, theoretical work in this research consisted of several 3D numerical analysis with the aim of understanding the constraint effects on the values of fracture toughness. Several parameters such as crack size and specimens geometry were analyzed to provide detailed information about constraint effects. This analysis revealed a variation of J integral and the parameter Q with the thickness of each specimen. New J-Q curves were drawn to describe the level constraint of plasticity and a more realistic set of values of fracture toughness. The results were compared with the ASME code, where it was possible to see that the code criteria are very conservative.

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Lee, Jonghee. "Fracture analysis of a propagating crack in a ductile material /." Thesis, Connect to this title online; UW restricted, 1996. http://hdl.handle.net/1773/7081.

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May, Daniel. "Transiente Methoden der Infrarot-Thermografie zur zerstörungsfreien Fehleranalytik in der mikroelektronischen Aufbau- und Verbindungstechnik." Doctoral thesis, Universitätsbibliothek Chemnitz, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-163082.

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In dieser Arbeit wurde eine neue fehleranalytische Methode zur industriellen Anwendung an neuen Technologien der Aufbau- und Verbindungstechnik entwickelt. Das Verfahren basiert auf der Wechselwirkung von thermischen Wellen und Defekten. Die Besonderheit ist dabei die Zerstörungsfreiheit, die Geschwindigkeit, das Auflösungsvermögen und die durch neueste IR-Detektoren erreichte Temperaturempfindlichkeit. Es wurden grundlegende Studien bezüglich Auflösung und parasitären Effekten bei der Anwendung unter industriellen Randbedingungen durchgeführt. Dabei wurde eine systematische Vorgehensweise bezüglich der Komplexität gewählt. Dies ermöglicht nun u.a. eine Vorhersage der zu erwartenden Prüfdauer zur Auflösung vergrabener Defekte, der Begrenzung der maximalen Anregungsimpulsbreite (bei gegebener Defekttiefe) und die quantitative Ermittlung des Einflusses einer Lackschicht. Methodisch kamen grundsätzlich Simulationen und vergleichende Experimente zum Einsatz. Es wurden spezielle Proben zur Isolierung und Klärung parasitärer Effekte verwendet. Letztlich konnte das Messsystem erfolgreich an industriellen Problemstellung demonstriert werden. Das entwickelte Messsystem zeichnet sich durch hohe Flexibilität aus. Verschiedene problemangepasste Anregungsquellen (interne und externe Anregung durch zahlreiche physikalische Effekte) kommen zum Einsatz. Das Messsystem besteht aus vier Hauptmodulen, der Differenzbild-Methode, der Impulsthermografie, und zwei Varianten der LockIn-Thermografie. Zusammen ist das System in der Lage, Voids, Delaminationen und Risse in verschiedenen Bereichen auch der modernen AVT sicher zu erkennen. Es werden dabei Temperaturauflösugnen bis zu 5 mK und laterale Auflösungen bis 17 µm erreicht. Diese Arbeit legt einen Grundstein für die Anwendung der thermischen Fehleranalytik in der Industrie, indem hier die Grenzen der IR-Messtechnik aufgezeigt und charakterisiert werden In this work a new failure analytical method for the industrial application of new technologies in electronic packaging has been developed. The developed method is based on the interaction of the thermal waves and defects. The special fature is non-destructive, speed, resolution and high temperature sensitivity due to latest IR-detectors. It fundamental studies regarding resolution and parasitic effects in the application were carried out cinsidering industrial conditions. Here, a systematic approach regarding the complexity has been selected. This now enables a prediction of the expected test period for detecting buried defects, limits for excitation pulse width (for a given defect depth) and the quantitative determination of the influence of parasitic paints. Methodically always simulations and comparative experiments were used. Simple samples for the isolation and purification of parasitic effects has been used. Finally, the measurement system has been successfully demonstrated on an industrial applications. The developed measurement system is characterized by high flexibility. Different problem-adapted excitation sources (internal and external excitation by numerous physical effects) are used. The measurement system currently consists of four main modules, the difference image method, the pulse thermography, and two variants of LockIn-thermography. Together, the system is capable of detecting voids, delaminations and cracks in various fields of electronic packageing. It will reach temperature resolutions up to 5 mK and lateral resolutions down to 17 µm. This work stes a foundation for the application of thermal failure analysis for industry by showing and charcterization the limits of IR imaging

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Cordeiro, Sérgio Gustavo Ferreira. "Contribuições às análises de fratura e fadiga de componentes tridimensionais pelo Método dos Elementos de Contorno Dual." Universidade de São Paulo, 2018. http://www.teses.usp.br/teses/disponiveis/18/18134/tde-05062018-104832/.

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O presente trabalho consiste no desenvolvimento de uma ferramenta computacional para análises de fratura e fadiga de componentes tridimensionais a partir de modelos geométricos de Desenho Assistido por Computador (CAD, acrônimo do inglês). Modelos de propagação de fissuras associados a leis empíricas de fadiga permitem a determinação da vida útil de peças mecânico-estruturais. Tais análises são de vital importância para garantir a segurança estrutural em diversos projetos de engenharia tais como os de pontes, plataformas off-shore e aeronaves. No entanto, a criação de modelos de análise a partir de modelos geométricos de CAD envolve diversas etapas intermediárias que visam a obtenção de malhas volumétricas adequadas. A grande maioria dos modelos de CAD trabalha com a representação de sólidos a partir de seu contorno utilizando superfícies paramétricas, dentre as quais se destacam as superfícies B-Splines Racionais Não Uniformes (NURBS, acrônimo do inglês). Para gerar malhas volumétricas é necessário que o conjunto de superfícies NURBS que descrevem o objeto seja \"estanque\", ou seja, sem lacunas e/ou superposições nas conexões das superfícies, o que não é possível garantir na grande maioria dos modelos constituídos por NURBS. As contribuições propostas no presente trabalho são aplicáveis a modelos baseados no Método dos Elementos de Contorno dual (MEC dual), os quais exigem apenas a discretização das superfícies do problema, ou seja, contorno mais fissuras. No intuito de criar os modelos de análise de maneira eficiente a partir dos modelos geométricos de CAD, desenvolveu-se uma estratégia de colocação que permite discretizar de maneira independente cada uma das superfícies NURBS que compõem os modelos geométricos sólidos. Com a estratégia proposta evitam-se as dificuldades no tratamento das conexões entre as superfícies sendo possível analisar modelos geométricos \"não estanques\". A implementação abrange superfícies NURBS, aparadas ou não, de ordens polinomiais quaisquer e elementos de contorno triangulares e quadrilaterais de aproximação linear. As equações integrais de deslocamentos e de forças de superfície são regularizadas e as integrais singulares e hipersingulares são tratadas pelo Método de Guiggiani. Fissuras de borda são inseridas nos modelos de análise a partir de um algoritmo de remalhamento simples baseado em tolerâncias dimensionais. O mesmo algoritmo é utilizado para as análises incrementais de propagação. Três técnicas de extração dos Fatores de Intensidade de Tensão (FIT) foram implementadas para os modelos baseados na Mecânica da Fratura Elástica Linear (MFEL), a saber, as técnicas de correlação, de extrapolação e de ajuste de deslocamentos. A extensão dessa última técnica para problemas tridimensionais é outra contribuição do presente trabalho. Os critérios da máxima taxa de liberação de energia e de Schöllmann foram utilizados para determinar o FIT equivalente e o caminho de propagação das fissuras. O ângulo de deflexão é determinado por um algoritmo de otimização e o ângulo de torção, definido para o critério de Schöllmann, é imposto no vetor de propagação a partir de uma formulação variacional unidimensional, definida sobre a linha de frente da fissura. Nos modelos de fadiga adota-se a MFEL e a equação de Paris-Erdogan para determinar a vida útil à propagação de defeitos preexistentes. Um procedimento iterativo foi desenvolvido para evitar a interpenetração da matéria após o contato das faces da fissura, permitindo análises de fadiga com carregamentos alternados. Como proposta para a continuidade da pesquisa propõe-se desenvolver formulações isogeométricas de elementos de contorno para analisar problemas de fratura e fadiga diretamente dos modelos geométricos de CAD, sem a necessidade de gerar as malhas de superfície. Um estudo numérico preliminar envolvendo uma versão isogeométrica do MEC dual baseada em NURBS e a versão convencional utilizando polinômios de Lagrange lineares e quadráticos foi realizado. A partir do estudo foi possível apontar as vantagens e desvantagens de cada formulação e sugerir melhorias para ambas. The present work consists in the development of a computational tool for fracture and fatigue analysis of three-dimensional components obtained from geometrical models of Computer-Aided Design (CAD). Crack propagation models associated with empirical fatigue laws allow the determination of residual life for structural-mechanical pieces. These analyses are vital to ensure the structural safety in several engineering projects such as in bridges, offshore platforms and aircraft. However, the creation of the analysis models from geometrical CAD models requires several intermediary steps in order to obtain suitable volumetric meshes of the problems. The majority of CAD models represent solids with parametric surfaces to describe its boundaries, which is known as the Boundary representation (B-representation). The most common parametric surfaces are Non-Uniform Rational B-Splines (NURBS). To generate a volumetric mesh it is required that the set of surfaces that describe the object must be watertight, i.e., without gaps or superposition at the surfaces connections, which is not possible to unsure using NURBS. The contributions proposed at the present thesis are applicable to models based on the Dual Boundary Element Method (DBEM), which require only the discretization of the surfaces of the problems, i.e., boundary and cracks. A special collocation strategy was developed in order to create the analysis models efficiently from the geometrical CAD models. The collocation strategy allows discretizing independently each one of the NURBS surfaces that compose the geometrical solid models. Therefore, the difficulties in the treatment of the surface connections are avoided and it becomes possible to create analysis models from non-watertight geometrical models. The implementation covers trimmed and non-trimmed NURBS surfaces of any polynomial orders and also triangular and quadrilateral boundary elements of linear order. The displacement and traction boundary integral equations are regularized and the strong and hypersingular integrals are treated with the Guiggiani\'s method. Edge cracks are inserted in the models by a simple remeshing procedure based on dimensional tolerances. The same remeshing approach is adopted for the incremental crack propagation analysis. Three techniques were adopted to extract the Stress Intensity Factors (SIF) in the context of Linear Elastic Fracture Mechanics (LEFM), i.e., the displacement correlation, extrapolation and fitting techniques. The extension of this last technique to three-dimensional problems is another contribution of the present work. Both the general maximum energy realise rate and the Schöllmann\'s criteria were adopted to determine the equivalent SIF and the crack propagation path. The deflection angle is obtained by an optimization algorithm and the torsion angle, defined for the Schöllmann\'s criterion, is imposed in the propagation vector through a one-dimensional variational formulation defined over the crack front line. The concepts of LEFM are adopted together with the Paris-Erdogan equation in order to determine the fatigue life of pre-existing defects. An iterative procedure was developed to avoid the self-intersection of the crack surfaces allowing fatigue analysis with alternate loadings. Finally, as suggestion for future researches, it was started the study of isogeometric boundary element formulations in order to perform fracture and fatigue analysis directly from CAD geometries, without surface mesh generation. A preliminary numerical study involving an isogeometric version of the DBEM using NURBS and the conventional DBEM using linear and quadratic Lagrange elements was presented. From the study it was possible to point out the advantages and disadvantages of each approach and suggest improvements for both.

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Aminpour, Mohammad Ali. "Finite element analysis of propagating interface cracks in composites /." Thesis, Connect to this title online; UW restricted, 1986. http://hdl.handle.net/1773/9993.

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Ahmadivala, Morteza. "Vers une planification optimale de la maintenance des structures existantes sur la base d'une analyse de fiabilité en fonction du temps." Thesis, Université Clermont Auvergne‎ (2017-2020), 2020. http://theses.bu.uca.fr/nondiff/2020CLFAC056_AHMADIVALA.pdf.

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Les structures de génie civil jouent un rôle important dans tout pays pour améliorer l'économie ainsi que le bien-être social et environnemental. Une défaillance indésirable peut avoir des impacts significatifs à différents niveaux pour le propriétaire de la structure et pour les utilisateurs. La fatigue est l'un des principaux processus de dégradation des structures en acier qui provoque une défaillance structurelle avant la fin de la durée de vie prévue. Pour éviter des défaillances imprévues à cause de la fatigue, une Gestion complète du Cycle de Vie structurelle (GCV) est nécessaire pour minimiser le coût du cycle de vie et maximiser la durée de vie structurelle. L'un des principaux objectifs du GCV peut être lié à l'optimisation de la planification de la maintenance structurelle. Atteindre cet objectif est une tâche difficile qui nécessite de relever certains défis tels que la prédiction de la performance structurelle dans l'incertitude, l'utilisation de données de Surveillance de l'État de la Structure (SES) pour réduire les incertitudes, la prise en compte du comportement de propagation des fissures pour des composants donnés, la fiabilité et la décision éclairée par les coûts, réalisation et effet des actions de maintenance, entre autres. En conséquence, les contributions suivantes sont prises en compte dans cette recherche pour améliorer les capacités du GCV structurel qui sont expliquées brièvement dans la suite. Développement d'une nouvelle méthode de fiabilité dépendent du temps pour l'analyse de la fiabilité en fatigue.Étudier l'efficacité des outils avancés de propagation des fissures pour étudier les problèmes de fissuration par fatigue indésirables et caractériser certaines actions de réparation possibles sur une étude de cas réel.Présentation des hypothèses et des étapes de simplification requises pour intégrer la méthode de fiabilité dépendent du temps proposé avec les modèles de propagation des fissures afin d'approximer la fiabilité de fatigue dépendent du temps.Comme première contribution de cette thèse, une nouvelle méthode de fiabilité dépendante du temps appelée AK-SYS-t est proposée. Cette méthode fournit un outil efficace et précis pour évaluer la fiabilité en fonction du temps d'un composant par rapport aux autres méthodes disponibles. AK-SYS-t relie la fiabilité en fonction du temps aux problèmes de fiabilité du système et tente d'exploiter les méthodes de fiabilité du système efficaces telles que AK-SYS pour l'analyse de la fiabilité en fonction du temps. Il convient de mentionner que l'analyse de la fiabilité en fonction du temps est nécessaire dans ce contexte, car la détérioration des performances (comme la fatigue) est un processus dépendant du temps associé à des paramètres dépendant du temps tels que le chargement de fatigue.Un autre sujet connexe est l'étude du phénomène de propagation des fissures avec des outils de modélisation avancés tels que la Méthode des Éléments Finis (MEF) et la méthode XFEM (eXtended Finite Element Method). À des fins d'illustration, la fissure à la racine d'une soudure d'angle est prise en compte (détail de fatigue courant dans les ponts avec platelage orthotropes). Une question importante étudiée ici est l'influence de la tension transversale dans la plaque de pont sur la direction de la propagation de la fissure. On montre comment l'augmentation de la tension transversale dans la plaque de pont peut modifier la propagation des fissures vers la plaque de pont. Ces fissures sont considérées comme dangereuses car elles sont difficiles à inspecter et à détecter. En fin de compte, XFEM est utilisé pour étudier l'efficacité de deux solutions de réparation possibles. (...) Civil engineering structures play an important role in any country for improving the economy together with the social and environmental welfare. An unwanted failure might cause significant impacts at different levels for the structure owner and for users. Fatigue is one of the main degradation processes on steel structures that causes structural failure before the end of the designed service life. To avoid unexpected failures due to fatigue, a comprehensive structural Life Cycle Management (LCM) is required to minimize the life-cycle cost and maximize the structural service life. One of the main objectives within the LCM can be related to optimizing the structural maintenance planning. Achieving this goal is a challenging task which requires to address some challenges such as predicting the structural performance under uncertainty, employing Structural Health Monitoring (SHM) data to reduce uncertainties, taking into account crack propagation behavior for given components, reliability and cost-informed decision making, and effect of maintenance actions among others. Accordingly, following contributions are considered in this research to improve the capabilities of structural LCM which are explained shortly in the sequel.Developing a new time-dependent reliability method for fatigue reliability analysis.Investigating the effectiveness of advanced crack propagation tools to study unwanted fatigue cracking problems and characterizing some possible repair actions on a real case study.Introducing the assumptions and simplification steps required to integrate the proposed time-dependent reliability method with crack propagation models to approximate the time-dependent fatigue reliability.As the first contribution of this thesis, a new time-dependent reliability method called AK-SYS-t is proposed. This method provides an efficient and accurate tool to evaluate time-dependent reliability of a component compared to other available methods. AK-SYS-t relates the time-dependent reliability to system reliability problems and tries to exploit the efficient system reliability methods such as AK-SYS towards time-dependent reliability analysis. It is worth mentioning that time-dependent reliability analysis is necessary in this context since the performance deterioration (such as fatigue) is a time-dependent process associated with time-dependent parameters such as fatigue loading.Another related topic is the study of crack propagation phenomenon with advanced modeling tools such as Finite Element Method (FEM) and eXtended Finite Element Method (XFEM). For illustration purposes, the crack in the root of a fillet weld is considered (common fatigue detail in bridges with orthotropic deck plates). One important issue investigated herein is the influence of the transversal tension in the deck plate on the direction of the crack propagation. It is shown how increasing the transversal tension in the deck plate may change the crack propagation towards the deck plate. Such cracks are considered dangerous since they are hard to inspect and detect. In the end, XFEM is used to investigate the effectiveness of two possible repair solutions.A supplementary contribution is related to introducing the required steps in order to integrate the newly developed time-depend reliability method with crack propagation problems through some applicational examples. This is a challenging task since performing the time-dependent reliability analysis for such problems requires a cycle-by-cycle calculation of stress intensity factors which requires huge computational resources. Therefore, the aim here is to introduce the assumptions and simplification steps in order to adopt the AK-SYS-t for fatigue reliability analysis. Accordingly, two examples are considered. (...)

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Rodrigues, Eduardo Alexandre. "Um modelo multiescala concorrente para representar o processo de fissuração do concreto." Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/3/3144/tde-10052016-113532/.

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Este trabalho propõe uma técnica de modelagem multiescala concorrente do concreto considerando duas escalas distintas: a mesoescala, onde o concreto é modelado como um material heterogêneo, e a macroescala, na qual o concreto é tratado como um material homogêneo. A heterogeneidade da estrutura mesoscópica do concreto é idealizada considerando três fases distintas, compostas pelos agregados graúdos e argamassa (matriz), estes considerados materiais homogêneos, e zona de transição interfacial (ZTI), tratada como a parte mais fraca entre as três fases. O agregado graúdo é gerado a partir de uma curva granulométrica e posicionado na matriz de forma aleatória. Seu comportamento mecânico é descrito por um modelo constitutivo elástico-linear, devido a sua maior resistência quando comparado com as outras duas fases do concreto. Elementos finitos contínuos com alta relação de aspecto em conjunto com um modelo constitutivo de dano são usados para representar o comportamento não linear do concreto, decorrente da iniciação de fissuras na ZTI e posterior propagação para a matriz, dando lugar à formação de macrofissuras. Os elementos finitos de interface com alta relação de aspecto são inseridos entre todos os elementos regulares da matriz e entre os da matriz e agregados, representando a ZTI, tornando-se potenciais caminhos de propagação de fissuras. No estado limite, quando a espessura do elemento de interface tende a zero (h ?0) e, consequentemente, a relação de aspecto tende a infinito, estes elementos apresentam a mesma cinemática da aproximação contínua de descontinuidades fortes (ACDF), sendo apropriados para representar a formação de descontinuidades associados a fissuras, similar aos modelos coesivos. Um modelo de dano à tração é proposto para representar o comportamento mecânico não linear das interfaces, associado à formação de fissuras, ou até mesmo ao eventual fechamento destas. A fim de contornar os problemas causados pela malha de elementos finitos de transição entre as malhas da macro e da mesoescala, que, em geral, apresentam diferenças expressivas 5 de refinamento, utiliza-se uma técnica recente de acoplamento de malhas não conformes. Esta técnica é baseada na definição de elementos finitos de acoplamento (EFAs), os quais são capazes de estabelecer a continuidade de deslocamento entre malhas geradas de forma completamente independentes, sem aumentar a quantidade total de graus de liberdade do problema, podendo ser utilizados tanto para acoplar malhas não sobrepostas quanto sobrepostas. Para tornar possível a análise em multiescala em casos nos quais a região de localização de deformações não pode ser definida a priori, propõe-se uma técnica multiescala adaptativa. Nesta abordagem, usa-se a distribuição de tensões da escala macroscópica como um indicador para alterar a modelagem das regiões críticas, substituindo-se a macroescala pela mesoescala durante a análise. Consequentemente, a malha macroscópica é automaticamente substituída por uma malha mesoscópica, onde o comportamento não linear está na iminência de ocorrer. Testes numéricos são desenvolvidos para mostrar a capacidade do modelo proposto de representar o processo de iniciação e propagação de fissuras na região tracionada do concreto. Os resultados numéricos são comparados com os resultados experimentais ou com aqueles obtidos através da simulação direta em mesoescala (SDM). A concurrent multiscale analysis of concrete is presented, in which two distinct scales are considered: the mesoscale, where the concrete is modeled as a heterogeneous material and the macroscale that treats the concrete as a homogeneous material. The mesostructure heterogeneities are idealized as three phase materials composed of the coarse aggregates, mortar matrix and the interfacial transition zone (ITZ). The coarse aggregates are generated from a grading curve and placed into the mortar matrix randomly. Their behavior is described using an elastic-linear constitutive model due to their significant higher strength when compared with the other two phases of the concrete. Special continuum finite elements with a high aspect ratio and a damage constitutive model are used to describe the nonlinear behavior associated to the propagation of cracks, which initiates in the ITZ and then propagates to the mortar matrix given place to a macro-crack formation. These interface elements with a high aspect ratio are inserted in between all regular finite elements of the mortar matrix and in between the mortar matrix and aggregate elements, representing the ITZ. In the limit case, when the thickness of interface elements tends to zero (h ?0) and consequently the aspect ratio tends to infinite, these elements present the same kinematics as the continuous strong discontinuity approach (CSDA), so that they are suitable to represent the formation of discontinuities associated to cracks, similar to cohesive models. A tensile damage model is proposed to model the nonlinear mechanical behavior of the interfaces, associated to the crack formation and also to the possible crack closure. To avoid transition meshes between the macro and the mesoscale meshes, a new technique for coupling non-matching meshes is used. This technique is based on the definition of coupling finite elements (CFEs), which can ensure the continuity of displacement between independent meshes, without increasing the total number of degrees of freedom of the problem. This technique can be used to couple non-overlapping and overlapping meshes.To make possible the concurrent multiscale analysis, where the strain localization region cannot be defined a priori, an adaptive multiscale model is proposed. In this approach the macroscale stress distribution is used as an indicator to properly change from the macroscale to the mesoscale modeling in the critical regions during the analysis. Consequently, the macroscopic mesh is automatically replaced by a mesoscopic mesh where the nonlinear behavior is imminent. A variety of tests are performed to show the ability of the proposed methodology in predicting the behavior of initiation and propagation of cracks in the tensile region of the concrete. The numerical results are compared with the experimental ones or with those obtained by the direct simulation in mesoscale (DSM).

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Klimis, Nikolaos. "Etude en laboratoire de l'attenuation des ondes longitudinales : application a la caracterisation geotechnique des roches." Paris, ENMP, 1987. http://www.theses.fr/1987ENMP0054.

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Etude en laboratoire de roche par essais non destructifs. On s'interesse aux differentes causes d'attenuation d'une onde longitudinale. Pour des roches plutoniques, la diffraction sur grains domine; pour un calcaire cristallin, l'attenuation suit l'evolution du champ microfissural; pour une roche poreuse, l'attenuation est influencee par la dimension du milieu poreux; l'attenuation augmente par ailleurs avec l'alteration; essai sur une aplite

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Papamichail, Chrysanthi. "Estimation of dynamical systems with application in mechanics." Thesis, Compiègne, 2016. http://www.theses.fr/2016COMP2284.

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Cette thèse porte sur inférence statistique, les méthodes bootstrap et l’analyse multivariée dans le cadre des processus semi-markoviens. Les applications principales concernent un problème de la mécanique de la rupture. Ce travail a une contribution double. La première partie concerne la modélisation stochastique du phénomène de la propagation de fissure de fatigue. Une équation différentielle stochastique décrit le mécanisme de la dégradation et le caractère aléatoire inné du phénomène est traité par un processus de perturbation. Sous l'hypothèse que ce processus soit un processus markovien (ou semi-markovien) de saut, la fiabilité du modèle est étudiée en faisant usage de la théorie du renouvellement markovien et une nouvelle méthode, plus rapide, de calcul de fiabilité est proposée avec l'algorithme correspondant. La méthode et le modèle pour le processus markovien de perturbation sont validés sur des données expérimentales. Ensuite, la consistance forte des estimateurs des moindres carrés des paramètres du modèle est obtenue en supposant que les résidus du modèle stochastique de régression, dans lequel le modèle initial est transformé, soient des différences de martingales. Dans la deuxième partie de la thèse, nous avons abordé le problème difficile de l'approximation de la distribution limite de certains estimateurs non paramétriques des noyaux semi-markoviens ou certaines fonctionnelles via la méthode bootstrap pondérée dans un cadre général. Des applications de ces résultats sur des problèmes statistiques sont données pour la construction de bandes de confiance, les tests statistiques, le calcul de la valeur p du test et pour l’estimation des inverses généralisés The present dissertation is devoted to the statistical inference, bootstrap methods and multivariate analysis in the framework of semi-Markov processes. The main applications concern a mechanical problem from fracture mechanics. This work has a two-fold contribution. The first part concerns in general the stochastic modeling of the fatigue crack propagation phenomenon. A stochastic differential equation describes the degradation mechanism and the innate randomness of the phenomenon is handled by a perturbation process. Under the assumption that this process is a jump Markov (or semi-Markov) process, the reliability of the model is studied by means of Markov renewal theory and a new, faster, reliability calculus method is proposed with the respective algorithm. The method and the model for the Markov perturbation process are validated on experimental fatigue data. Next, the strong consistency of the least squares estimates of the model parameters is obtained by assuming that the residuals of the stochastic regression model are martingale differences into which the initial model function is transformed. In the second part of the manuscript, we have tackled the difficult problem of approximating the limiting distribution of certain non-parametric estimators of semi-Markov kernels or some functionals of them via the weighted bootstrap methodology in a general framework. Applications of these results on statistical problems such as the construction of confidence bands, the statistical tests, the computation of the p-value of the test are provided and the estimation of the generalized inverses

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Staudinger, Ulrike. "Morphologie und Bruchverhalten von Block- und Multipfropfcopolymeren." Doctoral thesis, [S.l. : s.n.], 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1187261828675-34703.

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Mattrand, Cécile. "Approche probabiliste de la tolérance aux dommages." Phd thesis, Université Blaise Pascal - Clermont-Ferrand II, 2011. http://tel.archives-ouvertes.fr/tel-00738947.

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En raison de la gravité des accidents liés au phénomène de fatigue-propagation de fissure, les préoccupations de l'industrie aéronautique à assurer l'intégrité des structures soumises à ce mode de sollicitation revêtent un caractère tout à fait essentiel. Les travaux de thèse présentés dans ce mémoire visent à appréhender le problème de sûreté des structures aéronautiques dimensionnées en tolérance aux dommages sous l'angle probabiliste. La formulation et l'application d'une approche fiabiliste menant à des processus de conception et de maintenance fiables des structures aéronautiques en contexte industriel nécessitent cependant de lever un nombre important de verrous scientifiques. Les efforts ont été concentrés au niveau de trois domaines dans ce travail. Une méthodologie a tout d'abord été développée afin de capturer et de retranscrire fidèlement l'aléa du chargement de fatigue à partir de séquences de chargement observées sur des structures en service et monitorées, ce qui constitue une réelle avancée scientifique. Un deuxième axe de recherche a porté sur la sélection d'un modèle mécanique apte à prédire l'évolution de fissure sous chargement d'amplitude variable à coût de calcul modéré. Les travaux se sont ainsi appuyés sur le modèle PREFFAS pour lequel des évolutions ont également été proposées afin de lever l'hypothèse restrictive de périodicité de chargement. Enfin, les analyses probabilistes, produits du couplage entre le modèle mécanique et les modélisations stochastiques préalablement établies, ont entre autre permis de conclure que le chargement est un paramètre qui influe notablement sur la dispersion du phénomène de propagation de fissure. Le dernier objectif de ces travaux a ainsi porté sur la formulation et la résolution du problème de fiabilité en tolérance aux dommages à partir des modèles stochastiques retenus pour le chargement, constituant un réel enjeu scientifique. Une méthode de résolution spécifique du problème de fiabilité a été mise en place afin de répondre aux objectifs fixés et appliquée à des structures jugées représentatives de problèmes réels.

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Wen-ChengYeh and 葉文正. "Analysis of Crack Stress Intensity Factors and Stochastic Modeling of Crack Propagation." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/15526261102343701788.

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